As the invention is applicable to processes for the manufacture of pulp and paper or board, the invention will be described with reference to such processes.
Material producers, such as producers of pulp, paper, and board, are facing a rapidly changing world. They must meet increasing customer demands for specific grades in smaller batches. Products must meet quality demands specified within narrow limits. The increased environmental awareness is met by large efforts, not only to meet the more stringent demands and regulations set on their activity, but also to minimize the impact of their influence on the environment. Increased recycling within the production process as well as an increased use of reclaimed matter as raw material or additions to the process also put higher demand of the control and supervision of process. Additionally, these producers face a tougher competition, which can only be met with high-quality products.
The product quality is dependent on many parameters which are influenced by the raw material supply to the process, especially the consistency of raw material supply, the additions made during treatments, and the production conditions during these treatments. Thus, the product quality can change substantially throughout these processes. Therefore, methods for process control need to employ a system with sensors or devices for sampling throughout the process line and means to execute measurements and sampling, means to collect, collate, and process information obtained from the process, and means to execute corrective actions in the process.
For processes containing two or more unit processes, an overall optimization of the process is required. The optimization of the process includes an optimization of, for example, the product quality, the total economy, environmental aspects, energy consumption, maintenance of product equipment, etc. A typical example of an optimization is an optimization of the process in order to find optimal set-points for pumps, valves, etc. in the process to achieve a certain production volume during a given period of time under the given conditions. Thereby, the optimization includes the downloading of tank levels, temperatures, etc. on-line from a process computer system into a computer, and using the downloaded values together with algorithms adapted for the optimization and control of the process.
Normally, the values of a plurality of variables in the process will be measured by means of sensors and used as input to the algorithms for the optimization and control of the process. However, every now and then, such measurements will be incorrect due to the malfunctioning of the sensors. Some deviation between measured values and the predicted or real values of the variables in question can be tolerated, but when the error is above a certain level due to the malfunctioning of a sensor, that value should not be used in the optimization algorithm or in the control model of a control system, and the sensor in question should be replaced or repaired in order to make it possible to provide the control system with correct measurements.
Therefore, prior art has proposed different ways of tracking, identifying, and analyzing the malfunctioning or defective sensors. Prior art also suggests ways of tracking, identifying, and analyzing a malfunctioning or defective actuator or controller, or production unit or device.
However, prior art only suggests methods and/or systems that, upon detection of such malfunctioning, provide alarms, but do not propose specific steps or measures taken by the used software of a control system in order to upgrade or adjust itself with regard to malfunctioning sensors or process variations that initially might lead to one or more sensors being deemed as malfunctioning.